If you want to use port forwarding, you'll need to know that for every MathLink connection, two different ports are used.  The full syntax for TCPIP link names looks like this:

    LinkCreate["[email protected],[email protected]", LinkProtocol -> "TCPIP"]

8000 and 8001 are the port numbers while `1.2.3.4` is your IP address.  You can pass only a single port number to `LinkCreate` as the link name: in this case it will return a `LinkObject` containing the full name and the other port will be automatically selected.

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However, if you want to use SSH port forwarding, I strongly recommend using the [Remote Kernel Strategies][1] package.  It will save you a lot of trouble.  For example, it will automatically forward ports for all *three* links that Mathematica needs for Front End - Kernel communication.

Please see the presentation I linked to for more information on how the connection is made and why three links are needed.

  [1]: http://library.wolfram.com/infocenter/Conferences/7250/